1. Field of the Invention
This invention relates to the field of portable battery equipment, and more particularly, to a method and apparatus for charging portable batteries using synchronous rectification.
2. Description of the Related Art
To improve customer satisfaction with portable battery operated equipment, in particular notebook computers, it is desired to re-charge batteries as quickly as possible. The amount of time required to charge a battery depends on the chemical process as well as the battery charger power processing efficiency.
Re-chargers for portable batteries utilize switching regulators to regulate DC power input from a DC voltage source such as a battery or an AC to DC adapter. Switching regulators are typically classified into different configurations or "topologies." On such topology is the single-ended inductor circuit, consisting of relatively simple circuits where a switch determines whether the voltage applied to an inductor is the input voltage, V.sub.dc, or zero. In this manner, the output voltage is a function of the average voltage applied to the inductor. The switch may be implemented using various electronic components, for example, a power transistor, coupled either in series or parallel with the load. The regulator controls the turning ON and turning OFF of the switch in order to regulate the flow of power to the load. The switching regulator employs inductive energy storage elements to convert the switched current pulses into a steady load current. Power in a switching regulator is thus transmitted across the switch in discrete current pulses.
In order to generate a stream of current pulses, switching regulators typically include control circuitry to turn the switch on and off. The switch duty cycle, which controls the flow of power to the load, can be varied by a variety of methods. For example, the duty cycle can be varied by either (1) fixing the pulse stream frequency and varying the ON or OFF time of each pulse, or (2) fixing the ON or OFF time of each pulse and varying the pulse stream frequency. Which ever method is used to control the duty cycle, the switch in switching regulators is either OFF, where no power is dissipated by the switch, or ON in a low impedance state, where a small amount of power is dissipated by the switch. This generally results in fairly efficient operation with regard to the average amount of power dissipated.
One method that has been utilized to improve operational efficiency of voltage regulators employs synchronous rectification. In synchronous rectification, a pair of switches, which are connected in series between the input voltage and ground, are synchronized so that either the input voltage or ground is applied to the input of an inductor. The synchronous control of the switches provides improved efficiency compared to traditional circuits which employed a switch and a diode.
Certain switching regulators with synchronous rectification provide a positive output voltage, however, current can flow out of, or into the regulator's output. When input voltage is removed while current is flowing into the regulator's output, energy stored in the inductor will be discharged, creating excess voltage in the circuit. This over-voltage condition frequently results in destruction of circuit components.
There are several ways to prevent damage from reverse current. U.S. Pat. No. 5,731,694 issued to Wilcox et al. teaches a method and circuit for controlling reverse current in switching regulators with synchronous rectification. The Wilcox et al. patent optimizes protection during low load current efficiency but does not pertain to battery charging applications. Further, when power is removed and then reapplied to the Wilcox et al. device, the current overshoots a steady state value. In battery charging applications, this overshoot can cause undesirable oscillations in protection circuits, where a protection switch is tripped ON and OFF for several seconds. The oscillations result in pulse charging which greatly lowers the efficiency of the charging process. Additionally, the current overshoot decreases the useful life of battery charger components, which are fabricated with graphite having a lattice structure that breaks down when exposed to over-current conditions.
In view of the foregoing, it is desirable to provide a switching regulator with synchronous rectification for use in a battery recharger, wherein the switching regulator includes a control circuit which reduces or substantially eliminates current overshoot.